Wavelength division multiplexed (WDM) optical communication systems are known in which multiple optical signals, each having a different wavelength, are combined onto a single optical fiber to provide a WDM signal. Such systems typically include transmitters having a laser associated with each wavelength, a modulator configured to modulate the output of the laser to carry data, and an optical combiner to combine each of the modulated outputs. Receivers are also provided to demultiplex the received WDM signal into individual optical signals, convert the optical signals into electrical signals, and output data carried by those electrical signals.
Conventionally, WDM systems have been constructed from discrete components. For example, lasers and modulators have be packaged separately and provided on a printed circuit board. More recently, however, many WDM components have been integrated onto a semiconductor chip, also referred to a photonic integrated circuit (PIC). In particular, lasers and modulators have been integrated together on a common substrate along with the optical combiner.
Conventional optical modulators include Mach-Zehnder (MZ) modulators or interferometers, which typically include first and second waveguides or farms, the ends of which are optically coupled to one another. Electrodes may be provided on one or both of the first and second arms, such that biases or drive signals corresponding to the transmitted data are applied to the electrodes to thereto change the refractive index therein. As a result, the phase and/or amplitude of light in one or both of the arms can be modulated or varied in accordance with the transmitted data.
Typically, the bias supplied to the electrode is centered about a “null” of a transfer function associated with the MZ interferometer. If the lengths of the MZ arms or optical path lengths are not matched, changes in temperature of the MZ interferometer may cause the null to drift, resulting in data transmission errors. The amount of drift is proportional to a difference in optical path lengths between the two arms. By matching the optical path lengths, however, the null in the transfer curve remains substantially fixed. Differences in temperature between the two arms can also result in optical path length differences. Other sources of stress to the waveguides can also cause optical path length differences. Thus, to the extent the temperature of an MZ modulator may change, such temperature changes, and corresponding thermally induced optical path length changes, should be the same for each arm so that the changes are “common-mode”. Similarly, both arms should experience the same stress. When thermal or other stress to the waveguide arms is “common mode”, variations in the phase and/or amplitude of an optical signal propagating in one arm cancel out such variations in an optical signal propagating in the other arm when the optical signals are combined at the output of the MZ interferometer.
Accordingly, there is a need to provide MZ modulators in which variations in temperature or stress are “common mode” and are substantially the same for both arms. Moreover, there is a need to increase the density of optical components integrated onto a PIC and for the MZ arms to have substantially the same length.